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Kelvin Tyler
Reply with quote  #1 
Wolfgang. I have shifted our discussion on this topic to a new thread so as not to divert too much from the R-2R DAC subject.
To be absolutely clear I take it that the R2 and R1 resistors in my fig (a) are what you are calling Rf and Rg respectively. The 1M grid return resistor you show in your original circuit should have little effect if you make R1 1K and R2 your variable 50K. I assume the output impedance of your DAC is very low. That would make a good working setup to my mind. Yes, R2 is effectively in parallel with the choke as the the grid is close to being a virtual ground.
Now the missing 6dB of open loop gain. The gain of your original preamp with mu ~8 should be close to 18dB (8x) rather than the figure of 12bB (4x). As I said previously, the preamp would have to be seeing an effective load of about 7.5K to lower this gain to 12dB. I doubt if the combination of your output filter resistor and input impedance of the 300B amp would be as low as that.
I hope I am not misunderstanding the exact setup you are using and which you say works so well.
By the way, many thanks for 'energizing' me to renew my DAC. [smile]
Regards,
Kelvin
Wolfgang
Reply with quote  #2 

Kelvin, I need to measure the original circuit again. Obviously I didn’t remember the dB  correctly which I originally had measured when I finished the line stage many months ago. But what you wrote makes of course absolute sense.  This kind of stuff bugs me, so I will try to get clarity and post the results asap.

I also misunderstood the part about the 6dB as I thought you were talking about the 6dB which I lost through NFB when using  10k for Rg and 50k for Rf. But what you tried to point out was that the original gain of the line stage (no NFB) should be higher as “the preamp would have to be seeing an effective load of about 7.5K to lower this gain to 12dB”. Got it. My thinking was simply too preoccupied with the loss of 6dB in the NFB version and that's why I must have misunderstood what you tried to say.

And yes, R2=Rf, R1=Rg. Output impedance of the DAC is 625ohm.

I still don’t understand  why the smaller value for Rg has such an influence on the gain with the NFB if the reason for the gain loss would come from the 50k pot in combination with Ra (choke).

Kelvin Tyler
Reply with quote  #3 
W. I agree the operation of this setup can be a bit confusing.
If you simply connected a 50K resistor from the output to ground of your original preamp you would reduce the gain from 8x to about 6x. Now consider the case when the feedback is connected as in fig(a). The fraction of the output fed back is determined by the ratio of R2 to R1. R1 of course includes the output impedance of the DAC which you say is 625 ohms. If you add a further 1K, say, the feedback fraction would be 1.65/51.65 or ~0.03. This will not give very much loss of gain. As you reduce R2 this fraction will rapidly increase and the gain reduced. When R2 is at 10K, for example, the fraction fed back will be 1.65/11.65 or ~0.14 which will certainly be noticeable. On top of this there is also the output shunting effect of reducing R2. All in all a very dynamic situation, but you will see why increasing the value of R1 has such a marked influence
Often in a situation like fig (a) the open loop gain is very much higher as with a high-gain pentode or, in the extreme, an op-amp. Then R2/R1 is very close to the gain with feedback. Your 300B OTL is an example like this with an open loop gain of the order of 50dB or so.
Kelvin
Wolfgang
Reply with quote  #4 
Thanks, Kelvin. That really explains it!

Here are the numbers for the original version (no grid stopper, no output resistor to ground) at 1V input: 5,9V out/15,4dB open loop gain. This is how I used it with 15ohm Lowther/300B OTL DC.
For the 8ohm Lowther I need an output resistor to ground as some noise (I guess it’s mainly thermal noise) can be heard in very quiet passages:
With 8k2 output resistor: 3,2V out/10,1dB.
8k2 was the best compromise between no audible noise at sweet spot/ best transients.

With NFB at min setting and Rg=1k for 1V input: 3,8V out/11.6dB. Overall less noise and best transients. This is only one reason why I would always prefer the NFB version either as stand- alone volume control or in combination with digital volume control.

Wolfgang
Kelvin Tyler
Reply with quote  #5 
Thanks for posting your measured data, Wolfgang.
I suppose the other thing against using a high R1 value is indeed the effect of the Miller capacitance on your desirable transients or higher frequencies in general. The Miller effect should not be too severe at these low gain levels of course.
Your setup is a wonderful meld of 'ancient and modern'. I bet it gives cracking sonics. Amused to see you using a tube in your preamp which existed before even I did! [smile]. Is your 300B amp as beautiful as Ray P's elegant creation?
Kelvin
Wolfgang
Reply with quote  #6 
Your are right about the sound, Kelvin. It’s hard to believe how comparatively little efforts can make such a big difference when it comes to these often neglected or overlooked details.

As the word “transients” has been used so often in this context I thought it should be at least mentioned once why they are so important. As far as I know the transient component of a music instrument is essential for the human brain to recognize its timbre and put it into the right  context to the rest of the recording. Loudspeakers which cannot deliver fast transients will create the wrong impression of the size of an instrument, the wrong dimensions of a sound stage etc, and will show variations of this at higher or lower volume levels. Amps are of course as important as the speakers and the OTL power amps I know do exceptionally well in this respect.

I think Ray is among the few people who have posted their pics in this forum and who make very good looking amps – inside and outside. I try my best but cannot reach that level of perfection but I also don’t want to spend the extra money for getting the holes professionally drilled, anodizing, and suchlike things. I am also side tracked too soon by too many ideas about changing things a little, “improvements” as I call it. That’s why my amps look rather humble from outside and packed from inside.
I think we can only benefit from your experience, Kelvin, which you must have gained over decades. I do.

Attached Images
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Kelvin Tyler
Reply with quote  #7 
Excellent work, Wolfgang. I like the chassis/boxes you have used and note the separate transformer enclosure.
It is good to see in concrete form many of the improvements you have made to the original design. I can recognize most of them. It is inevitable that when you make so many additions the project grows like 'Topsy', but to my eye you have added your mods very well.
This DIY audio is great fun and a little extra effort can bring very pleasing rewards.
Your kind words are much appreciated.
Kelvin
Dave_CA
Reply with quote  #8 
What speakers are each of the posters in this thread and the R-2R DAC thread using? 
Wolfgang
Reply with quote  #9 
8 ohm Lowther DX4 in Medallions II (adjusted compression chamber for DX4) and 15 ohm Lowther DX4 in C-Horns.
Kelvin Tyler
Reply with quote  #10 
I would love a speaker setup like that, but my space restrictions are too severe. Mostly using Fostex FE166En in 17 litre front ported BR cabinets with occasional Wharfedale 150 SW sub support. Hardly an optimal setup, but still sounds well with excellent TS 300B PP amp and Masterpiece.
There are suspicions from the distaff side that a clandestine experimental BH project may be taking shape in my workshop/cupboard.[wink].
Kelvin
Ray P
Reply with quote  #11 
So can you tell us more about your speaker project Kelvin - I promise not to tell anyone.

I use 15ohm Lowther EX4s in AllFun horns with my 300B SE-OTLs. I have toyed with the idea of trying some OBs and have a pair of Fostex FE167 units I might experiment with.

Anyway, I noticed this thread over on DIY Audio;

http://www.diyaudio.com/forums/solid-state/309981-variable-gain-volume-control.html

Ray
Kelvin Tyler
Reply with quote  #12 
Hi Ray-P. Thanks for the link re variable gain volume controls.Will read in detail later, but looks interesting. I have a feeling that W. quite likes what he has at the moment, but one can never be sure that a new 'improvement' will not emerge at any time! [smile].
Let me say at the outset that my detailed experience with BLH technology is close to zero. In the past, like many others, I followed Gilbert Briggs' (Wharfedale) suggestions on speaker acoustics, cabinets, etc. Recently I decided to venture into the theory and practice of BLH systems and found that, as with so many things on the web, very strong 'filters' had to be applied to get a coherent picture.
Now my listening room geography does not give easy access to corners so I decided to focus on front ported designs ( possibly in error here). I acquired a couple of Fostex FE126En drive units and then considered cabinet possibilities of not too great a size. The Fostex hybrid cabinet did not look right and it seemed to attract mostly negative comments. So, being of a somewhat perverse nature, I knocked up a couple of boxes to this design, but with removable side panels so that modifications could be made. With brand new drive units the sound was quite harsh, but this was to some extent alleviated by the addition of damping material and the passage of time. There remained some nasty medium/high frequency resonances however and I did not find the bass-reflex port added much to the overall performance. I have not given up completely with these boxes yet as I have a few more experiments to make and they are quite (too?) compact; they may just make it to November 5th!
Meanwhile I have switched my efforts to self-building a couple of Madisound BK-12 boxes. I trust they will do a bit better.
Incidentally, I found it very informative to use a small sound level meter to 'see' what was emerging from the front cone area and from the horn mouth at different sine wave frequencies.
Very stumbling steps on this one I confess.
Kelvin
Gary Kemp
Reply with quote  #13 
Kelvin I'll soon have my pair of FE206's (backloaded horns) back - you'll have to come hear them. 

Quote:
Originally Posted by Kelvin Tyler
Hi Ray-P. Thanks for the link re variable gain volume controls.Will read in detail later, but looks interesting. I have a feeling that W. quite likes what he has at the moment, but one can never be sure that a new 'improvement' will not emerge at any time! [smile].
Let me say at the outset that my detailed experience with BLH technology is close to zero. In the past, like many others, I followed Gilbert Briggs' (Wharfedale) suggestions on speaker acoustics, cabinets, etc. Recently I decided to venture into the theory and practice of BLH systems and found that, as with so many things on the web, very strong 'filters' had to be applied to get a coherent picture.
Now my listening room geography does not give easy access to corners so I decided to focus on front ported designs ( possibly in error here). I acquired a couple of Fostex FE126En drive units and then considered cabinet possibilities of not too great a size. The Fostex hybrid cabinet did not look right and it seemed to attract mostly negative comments. So, being of a somewhat perverse nature, I knocked up a couple of boxes to this design, but with removable side panels so that modifications could be made. With brand new drive units the sound was quite harsh, but this was to some extent alleviated by the addition of damping material and the passage of time. There remained some nasty medium/high frequency resonances however and I did not find the bass-reflex port added much to the overall performance. I have not given up completely with these boxes yet as I have a few more experiments to make and they are quite (too?) compact; they may just make it to November 5th!
Meanwhile I have switched my efforts to self-building a couple of Madisound BK-12 boxes. I trust they will do a bit better.
Incidentally, I found it very informative to use a small sound level meter to 'see' what was emerging from the front cone area and from the horn mouth at different sine wave frequencies.
Very stumbling steps on this one I confess.
Kelvin
Wolfgang
Reply with quote  #14 
The Ayre AX-5 (see link in diyAUDIO thread)does something else.“Variable gain transconductance” changes the transconductance of the JFETs of the input stage by selecting different resistor values with the volume control rather than using a pot as a simple voltage divider. The goal is better S/N ratio at lower volume levels. The Ayre doesn’t use NFB at all.

The volume control in the case of the line stage controls the input signal simply by adding NFB. What’s special in this case is that the anode is choke loaded which gives the signal a better dynamic behavior than resistor loaded and this somehow must have some impact on the input signal through NFB. That’s at least my explanation at this point. This form of volume control does something else besides changing the volume (at least this is what I can hear in my system): it creates a very precisely balanced virtual sound stage and separation between instruments with incredibly detailed treble which I know only from highend preamps with hundreds of electronic parts and several stages of amplification, local NFB, each stage with its own psu etc. But at the same time the RCA 26 line stage doesn't suffer from the side effects of such like amps which often can sound sterile or anemic.
Kelvin Tyler
Reply with quote  #15 
Have now read up a bit on the AX-5 amp. Wow! A gain control with overtones of the double-overhead-camshaft setup from a Jaguar XK auto engine, complete with attendant gear shift noises! JFET transconductance alteration rather reminds me of radio AVC systems using vari-mu pentodes. Glad to read that the AX-5 sounds so good.
Wolfgang. Your comparison of some complex 'high end' preamps with your 26 based system brings to mind the old adage 'More is Less'. I suppose an absolute purist might wish to add a cathode follower to your preamp in order to present a constant output impedance, but then again 'If it ain't broke don't fix it'! [smile]
Gary. Thanks. Let me know when you recover your FE206 speakers.
Kelvin
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